Some types of Ethernet transceivers, for example gigabit Ethernet transceivers are configured to transmit data over copper cabling. Take, for example, 1000BASE-T Ethernet transceivers. Conventional 1000BASE-T Ethernet transceivers include a PHY (physical interface transceiver) having four transmit and receive sections for implementing full-duplex physical layer signaling. Gigabit Ethernet speed can be achieved by connecting the PHY to a Category-5 (CAT-5) type copper cable or higher having four different twisted-pair links. The transmit/receive sections of the PHY are coupled to different ones of the twisted-pair links. Each twisted-pair link has an individual capacity of approximately 250 Mbps (Mega bits per second) for cable lengths of about 100 m or less. Thus, a total link capacity of approximately 1000 Mbps is available when all four links are employed.
During operation, eight bits of data are transmitted each transmission window over the four twisted-pair links using a 125 MHz symbol rate. The twisted-pair links employ a five-point, 1-dimensional (1D) constellation having five symbol values (−2, −1, 0, +1, +2) encoded using three bits. The four twisted-pair links collectively form a 4-dimensional (4D) constellation where each twisted-pair link represents one of the dimensions. Thus, a 4D symbol is transmitted at gigabit Ethernet speeds by encoding eight bits of data into four symbols and transmitting each of the four symbols over a different one of the twisted-pair links in parallel.
Gigabit Ethernet transceivers are not always connected to cabling having gigabit transmission bandwidth. For example, Cateogry-3 (CAT-3) or other comparable 2-pair type of cabling cannot support gigabit Ethernet speeds. Conventional Ethernet PHYs have an auto-negotiation feature for detecting the type of cabling and configuring the PHYs accordingly. Typically, the data transmission rate of a gigabit Ethernet PHY is reduced from 1000 Mbps (1000BASE-T) to 100 Mbps (100BASE-TX) or 10 Mbps (10BASE-TX) when sub-gigabit capacity cabling is connected to the PHY. However, each twisted-pair link connected to the PHY still has a capacity of approximately 250 Mbps at cable lengths of about 100 m or less. Transceiver performance is not optimized when the PHY operates at 100 Mbps or 10 Mbps even though each twisted-pair link has a much greater capacity.